BEIJING, May 18 (Reuters) - Two major Chinese coal-fired power generators have banned their purchasing departments from buying thermal coal above certain prices on spot markets as they expect prices to fall in the coming months.

China Huaneng Group and China Huadian Corp Ltd, blamed the rally on coal prices since mid-April for “irrational market expectations”, according to internal notices from the two firms reviewed by Reuters and confirmed by their executives.

Benchmark thermal coal futures on the Zhengzhou Commodity Exchange have gained more than 7 percent over the past month.

In the notices, Huaneng and Huadian also called on customs to speed up import checks to support coal imports and on local governments to boost coal output.

Huaneng and Huadian did not immediately response to a fax on the issue. (Reporting by Muyu Xu, Meng Meng and Josephine Mason)